I just purchased a 990/7500 combination about a week ago and have been experiencing a buzz. It has about a 60 Hz fundamental. Some experimentation shows that:
1. It comes and goes when the light engine in the television powers up/down. (Samsung HL-T6187S, rear projection LED light source 61").
2. Disconnecting various cables between the 990 and the TV make no difference in the hum
3. Powering the Samsung from an extension cord from another room gets rid of the hum.

The question now is whether I can filter the power sufficiently to eliminate the hum or I will have to run another power circuit.

Any other possible solutions appreciated.

Steve Mahan

Outlaw 990/7500/LFM1+
Adcom 555 II (in reserve)
Comcast digital Hi-Def cable DVR
Kef 107 (down for woofer surrounds)
Kef XQ-2c center (waiting for arrival)
generic CD and DVD players
still seeking surround speakers... (suggestions appreciated)